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Retirement Plans and Other Postretirement Benefits
 3 Months Ended
Mar. 31, 2019
Retirement Benefits [Abstract]  
Retirement Plans and Other Postretirement Benefits Retirement Plans and Other Postretirement Benefits
 
Pinnacle West sponsors a qualified defined benefit and account balance pension plan, a non-qualified supplemental excess benefit retirement plan, and an other postretirement benefit plan for the employees of Pinnacle West and our subsidiaries.  Pinnacle West uses a December 31 measurement date for its pension and other postretirement benefit plans.  The market-related value of our plan assets is their fair value at the measurement dates.

The following table provides details of the plans’ net periodic benefit costs and the portion of these costs charged to expense (including administrative costs and excluding amounts capitalized as overhead construction or billed to electric plant participants) (dollars in thousands):

 
Pension Benefits
Other Benefits
 
Three Months Ended 
 March 31,
 
Three Months Ended 
 March 31,
 
2019
 
2018
 
2019
 
2018
Service cost — benefits earned during the period
$
12,543

 
$
14,213

 
$
4,714

 
$
5,105

Non-service costs (credits):
 
 
 
 
 
 
 
Interest cost on benefit obligation
34,352

 
31,007

 
7,526

 
7,101

Expected return on plan assets
(42,893
)
 
(45,667
)
 
(9,603
)
 
(10,520
)
  Amortization of:
 

 
 
 
 

 
 

  Prior service cost (credit)
—

 
—

 
(9,455
)
 
(9,461
)
  Net actuarial loss
11,239

 
7,782

 
—

 
—

Net periodic benefit cost (credit)
$
15,241

 
$
7,335

 
$
(6,818
)
 
$
(7,775
)
Portion of cost (credit) charged to expense
$
8,244

 
$
2,242

 
$
(4,817
)
 
$
(5,605
)

 
Contributions
 
We have made voluntary contributions of $90 million to our pension plan year-to-date in 2019. The minimum required contributions for the pension plan are zero for the next three years. We expect to make voluntary contributions up to a total of $350 million during the 2019-2021 period. We do not expect to make any contributions over the next three years to our other postretirement benefit plans.